In certain aspects, the present invention may be regarded as an important improvement in electrical switches of the general construction disclosed and claimed in prior U.S. patents, assigned to the assignee of the present application. Such prior U.S. patents include Hoy and Hazek No. 3,223,794, issued Dec. 14, 1965; Hoy and Hazek No. 3,319,016, issued May 9, 1967; and Hoy and Hazek No. 3,378,654, issued Apr. 16, 1968.
In the general switch construction of such prior patents, some of the fixed contact members take the form of one or more conductive contact plates, mounted on the inner side of an insulating plate or wall, closing one side of a casing. A conductive contactor is movable in the casing and is engageable with the contact plate, and may also be engageable with other contact elements, such as contact points. The contactor is mounted on a carriage which is movable along a predetermined path in the casing. The carriage is operated by some external means, such as an operating lever. A spring may be provided between the carriage and the contactor, for resiliently biasing the contactor toward the contact plates and any other contact elements. In some cases, the switch may have a plurality of contact plates. The switch may also have a plurality of contactors, in some cases.
In the general switch construction of such prior patents, the contact plate or plates may have one or more contact rails, along which a contactor is slidable. The contactor may have a point or other rider, adapted to ride along two adjacent contact rails on the same contact plate or on separate contact plates.
In the general switch construction of such prior patents, the contact plates are connected to corresponding terminal bars or prongs, mounted on the outer side of the insulating wall and connected to the plates by means of rivets. Each rivet extends through a mounting hole in a contact plate, an aligned opening in the insulating wall, and an aligned mounting hole in the corresponding terminal bar. The rivet may have a head engaging the contact plate and may be swaged or upset, so as to clamp the contact plate and the terminal bar against the inner and outer sides of the insulating wall. It has been the practice in such prior switches to swage the rivet with such force that the shank of the rivet is bulged to produce an interference fit between the shank and the mounting hole in the terminal bar or prong. The interference fit establishes and maintains a good, low-resistance electrical contact between the terminal rivet and the terminal bar.
However, in certain cases, some difficulty has been experienced in establishing and maintaining a good, low-resistance electrical contact between the terminal rivet and the contact plate. The prior switches have relied upon the clamping pressure between the rivet head and the contact plate, to achieve good electrical contact. In certain cases, difficulties have been experienced in reliably establishing maintaining such clamping pressure, because there has been a tendancy for looseness to develop between the rivet head and the contact plate, due mostly to variations in the thickness of the insulating wall. Such thickness variations may be caused by changes in temperature and humidity, and also by difficulties in maintaining sufficiently close dimensional tolerances in the initial thickness of the insulating wall, particularly when such wall is in the form of an insulating plate stamped from insulating sheet material.
In the prior switch construction, any looseness between the rivet head and the contact plate tends to increase the resistance of the electrical contact therebetween. The increased resistance may cause overheating of the switch, particularly in those instances in which a high electrical current is being carried. In some cases, the overheating may be sufficiently great to damage the switch.
The present invention deals with the problem of preventing such overheating of electrical switches of this general construction, utilizing conductive contact plates.